Another possibility:
Unless you land on a completely empty paved hex, the ground is rarely going to be completely flat. There are things that stick up out of it (boulders, light posts, trees, civilian vehicles, statuary, etc etc) that while under normal circumstances are easy to avoid and don't impair line of sight or movement, can do damage if you land on it wrong. So you attempted to do a DFA, missed, landed badly and fell forward or back landing in the prone position. And you fell down just right that your mech's head (and the pilot in it) gets impaled on one of these normally trivial projections.
Heck, since we're talking a DFA, your "miss" could have been your mech headbutting your target's knee or shoulder or something instead of landing on your target feet first. A well done landing on the ground is usually depicted as the mech bending its knees to absorb the impact, and the torso (with head!) leaning forward for balance reasons. But if the space you're leaning into is already occupied by your target...